  from functools import wraps
  def singleton(cls):
      """装饰类的装饰器"""
      instances = {}
      @wraps(cls)
      def wrapper(*args, **kwargs):
          if cls not in instances:
              instances[cls] = cls(*args, **kwargs)
          return instances[cls]
      return wrapper
  @singleton
  class President:
      """总统(单例类)"""
      pass